Process for preparing calcium carbonate

ABSTRACT

Inexpensive spindle- or rice-like calcium carbonate giving useful properties for brightness, opacity and wire abrasion resistance as a paper filler is provided by taking advantage of the causticization step. When a white liquor is used for slaking a quick lime, the quick lime used in the slaking reaction of a first step has a calcium carbonate content of 10% by weight or less and is slaked with the white liquor at a concentration of 20 to 60% to prepare a milk of lime. When a green liquor is used for slaking a quick lime, the quick lime used in the slaking reaction of a first step has a calcium carbonate content of 0.1 to 10% by weight and is slaked with the green liquor at a concentration of 20 to 60% to prepare a milk of lime. In a second step, causticization reaction takes place at a green liquor loading rate of 0.02 to 0.5 cc (green liquor)/min/g (quick lime) at a reaction temperature of 20 to 105° C. to afford spindle- or rice-like calcium carbonate.

FIELD OF THE INVENTION

The present invention relates to processes for preparing calciumcarbonate giving useful performances as a paper filler in thecausticization step of the sulfate or soda pulp process, and morespecifically processes for preparing calcium carbonate giving usefulperformances as a paper filler by defining the quick lime used andslaking and causticization conditions or other factors.

PRIOR ART

Normally, a filler is internally added to printing or writing papers forthe purpose of improving brightness, opacity, smoothness, writingsuitability, touch, printability or other properties. Such papers aremade by the so-called acid process at about pH 4.5 or the so-calledneutral process at a neutral to weakly alkaline pH of 7 to 8.5 usingtalc, clay, titanium oxide or the like as a filler. In the neutralprocess, domestic calcium carbonate can be used in place of expensiveimported talc or clay. In recent years, neutral papers obtained by theneutral process have attracted interest because of the papers'storability, and the number has been growing and will becomeincreasingly widespread because of many advantages including paperquality, costs, environmental influences.

A feature of recent demands for paper is a significant growth in thefield of leaflets, catalogs, pamphlets, direct mails or the like incommercial printing and in the field of books related to computer,multimedia and family computer to meet the development of theinformation age, magazines, collections of photographs, mooks, comics orthe like in publishing. Thus, paper users increasingly desire to lowerthe cost and seek downgraded and lighter paper.

As the demands for inexpensive and light neutral paper increase asdescribed above, the role of calcium carbonate as a filler will becomevery important. Calcium carbonate used as a filler in neutral papersincludes heavy calcium carbonate obtained by mechanically dry- orwet-grinding a natural limestone and precipitated calcium carbonatechemically synthesized (synthetic calcium carbonate).

However, in heavy calcium carbonate obtained by grinding a naturallimestone by a mill such as ball mill it is difficult to control theshape and it severely abrades plastic wires during paper makingprocesses when it is used as an internal filler. Moreover, normalfine-quality papers or coated papers prepared with such a filler areinsufficient in bulk, brightness, opacity, smoothness, writingsuitability, touch, printability and other properties.

Recent advances in weight saving make the above problem graver.High-specific surface fillers (e.g. pulverized silica, white carbon),highly refractive fillers (e.g. titanium dioxide) or precipitatedcalcium carbonate (synthetic calcium carbonate) have been so far used ascommon means for improving the opacity of light printing papers.

The precipitated calcium carbonate is known to be prepared by (1) areaction between carbon dioxide gas and a milk of lime obtained from acalciner of lime or the like, (2) a reaction between ammonium carbonateand calcium chloride in the ammonia soda process, (3) a reaction betweena milk of lime and sodium carbonate in the causticization of sodiumcarbonate, etc. Among these processes, (2) and (3) have been lessexamined with respect to controlling the shape of the resulting calciumcarbonate because both reactions produce it as a by-product and are nowbeing replaced by novel formats for obtaining the main products. On theother hand, (1) has been widely studied with respect to techniques forpreparing calcium carbonate in various shapes and have actually createdsome examples prepared on-site in paper factories, partly because thereaction system is relatively simple (water, slaked lime, carbon dioxidegas). However, the production costs of this process are very highbecause calcium carbonate is the sole product and so it can not satisfyusers' demands for cost-saving and can not be used, or at most in agreatly limited amount, for inexpensive types of papers.

A possible alternative is to use calcium carbonate by-produced duringthe causticization step for recovering and regenerating a digestingagent in a kraft pulp process, as a paper making material.

In the sulfate or soda pulp process, wood is digested with a mixedchemical solution of sodium hydroxide and sodium sulfide at hightemperature under high pressure to isolate cellulose. Cellulose isseparated as a solid phase and purified into pulp, while the chemicalsolution and other elements than cellulose eluted from the wood arerecovered as a pulp waste liquor (black liquor) and concentrated andburned. During that time, the eluted elements from the wood arerecovered as a heat source while inorganic matters based on sodiumcarbonate and sodium sulfide in the chemical solution are recovered anddissolved in water or a dilute chemical solution called weak liquor inwhich are dissolved a part of white liquor components generated whencalcium carbonate sludge formed by the reaction shown below is washed togive a green liquor. This green liquor is mixed with a quick lime toproduce calcium carbonate by the reactions (1) and (2):

CaO+H₂O→Ca(OH)₂  (1)

Ca(OH)₂+Na₂CO₃→CaCO₃+2NaOH  (2)

This calcium carbonate can be prepared at very low cost because it is aby-product of the preparation of the main product white liquor.Moreover, it can be expected to improve the reactivity of the abovereactions (1) and (2) and the clarity of the white liquor and to reducewaste, because the inside of the system can be clarified and thecirculating lime can be highly purified by extracting calcium carbonatefrom the calcium circulating cycle (calcium carbonate, quick lime,slaked lime) in the causticization step which is conventionally a closedsystem.

However, it was difficult to control the shape of the thus obtainedconventional calcium carbonate and it was always amorphous or massiveincluding various shapes such as cube or hexahedron with large particlediameters similar to conventional heavy calcium carbonate and normalfine-quality paper or coated papers prepared with such a filler wereinsufficient in bulk, brightness, opacity, smoothness, writingsuitability, sense of touch, printability or other properties. Withrecent large-scale paper-making machines producing paper at high speed,serious problems in plastic wire abrasion resistance and wet end yieldalso occurred.

Thus, it was difficult to efficiently and inexpensively prepare calciumcarbonate, which is useful as a filler or pigment to give a good plasticwire abrasion resistance during paper making processes; it was difficultalso to produce highly opaque fine-quality paper or coated paper whilemaintaining printing quality.

Considering the above situation, an object of the present invention isto provide inexpensive and light calcium carbonate with a controlledshape self-generated in the causticization step, which gives a good wireabrasion resistance during paper making processes and can be used toproduce fine-quality paper or coated paper having high opacity andexcellent printability or other properties.

SUMMARY OF THE INVENTION

As a result of careful studies to overcome the above problems, we foundthat they can be solved by slaking a quick lime containing calciumcarbonate at a specific level or less with a white liquor to prepare amilk of lime and taking advantage of the causticization step of thesulfate or soda pulp process to continuously add a green liquorgenerated in the causticization step of the sulfate or soda pulp processin an amount comparable to that required for preparing a white liquor inconventional procedures to said milk of lime at a controlled loadingrate and reaction temperature, and accomplished the present invention onthe basis of this finding. According to the process of the presentinvention, the shape of calcium carbonate can be controlled withoutsignificant change in the conventional causticization step to preparecalcium carbonate in the form of spindle- or rice-like particles havinga minimum diameter of 0.3 to 1.5 μm and a maximum diameter of 0.5 to 7μm, which can improve brightness, opacity and wire abrasion resistanceas a paper filler at a much lower cost than calcium carbonate obtainedby the conventional reaction between a milk of lime and carbon dioxidegas. Additionally, the duration of the kiln operation can be shortenedby extracting calcium carbonate from the causticization step or even bestopped depending on the amount of calcium carbonate extracted from thestep, thus saving on the cost of the entire causticization step.

DETAILED DESCRIPTION OF THE INVENTION

The quick lime used during the slaking reaction of the first step of thepresent invention may be the calcination product of a limestone based oncalcium carbonate and/or calcium carbonate generated during conversionof sodium carbonate into sodium hydroxide in the causticization step ofthe sulfate or soda pulp process. The calciner used here may be anyapparatus for converting calcium carbonate into a quick lime (calciumoxide) such as Beckenbach kiln, Meltz kiln, rotary kiln, Kunli kiln, KHDkiln, fluidized calciner, vertical mixing kiln.

Among impurities in the resulting calcium carbonate, coloring elements(Fe, Mn, etc.) must be especially controlled by suitably selecting aquick lime derived from a feed limestone containing less amounts ofcoloring elements to meet the purpose of the product paper. In the caseof a quick lime recalcined in a rotary kiln or fluidized calciner or thelike during the causticization step, a feed limestone containing lesscoloring elements may be supplied to the calcium circulating cycle ofthe causticization step or a controlled amount of the quick limeresulting from the calcination thereof may be used, depending on theconditions such as the ratio between calcium carbonate extracted outsidethe system and calcium carbonate recirculating in the system.

The calcium carbonate content in the quick lime is 0.1 to 10% by weighton the basis of the weight of the quick lime. If it exceeds 10% byweight, amorphous or massive calcium carbonate is produced, which has alow wire abrasion resistance and can not produce light coated paper withan intended quality. The content of 0.1% or less is uneconomic, becausethe energy required for calcination increases significantly or thecalciner must be specially designed. The particle size of the quick limeis not specifically limited, but preferably 0.01 to 30 mm. If it is 0.01mm or less, pulverization adds to the cost and dust occurs or transferis troublesome. If it is 30 mm or more, homogeneous mixing can not beobtained by agitation during slaking.

When a white liquor is used for slaking the quick lime, it should beused at 80 to 160 g/L (expressed as Na₂O in the same way as below),preferably 100 to 150 g/L in terms of total alkali or 30 g/L or less,preferably 25 g/L or in terms of Na₂CO₃. If the total alkali is lessthan 80 g/L, the concentration of the final white liquor is lowered andshould be regulated before it is used for digestion. If the total alkaliis more than 160 g/L, however, the resulting calcium carbonate has a lowwire abrasion resistance without attaining an intended paper quality. IfNa₂CO₃ is higher than 30 g/L, the resulting calcium carbonate also has alow wire abrasion resistance without attaining the desired paperquality. The lime level during slaking here should be 0.5 to 60% byweight, preferably 3.5 to 55% by weight on the basis of the quick limebefore slaking. If it exceeds 60% by weight, the viscosity of the liquorbecomes too high to practically agitate. If it is lower than 0.5% byweight, the productivity of calcium carbonate is unrealisticallylowered.

When a green liquor is used for slaking the quick lime, it may bederived from the causticization step of the conventional sulfate or sodaprocess and it should be used at 80 to 160 g/L (in which Na₂CO₃represents 65 to 130 g/L), preferably 100 to 150 g/L (in which Na₂CO₃represents 85 to 130 g/L) in terms of total alkali. If the total alkaliis less than 80 g/L (in which Na₂CO₃ represents 65 g/L), theconcentration of the final white liquor is lowered and should beregulated before it is used for digestion. If the total alkali is morethan 160 g/L (in which Na₂CO₃ represents 130 g/L), however, theresulting calcium carbonate has a low wire abrasion resistance withoutattaining an intended paper quality. The lime level during slaking hereshould be 20 to 60% by weight, preferably 25 to 55% by weight on thebasis of the quick lime before slaking. If it exceeds 60% by weight, theviscosity of the liquor becomes too high to practically agitate. If itis lower than 20% by weight, massive calcium carbonate particles aregenerated with low wire abrasion resistance, which can not attain thedesired paper quality.

The quick lime and the white liquor or green liquor may be mixed using ameans appropriately selected from conventional agitating blade- orpump-type extruders, kneaders and blenders depending on the viscosity ofthe liquor or slurry during mixing (see Handbook of Chemical Engineeringpublished by Maruzen, Mar. 18, 1988).

The slaking temperature and period are closely related to each other. Ashort period suffices if the temperature of the aqueous solution usedfor slaking is high, while a long period is required if the temperatureis low. The period is appropriately determined to meet the temperaturecondition of the quick lime used during slaking. As a standard, thereaction may be continued until the temperature rises due to heatgeneration during slaking stops. Practically, slaking is effective at atemperature as high as possible.

The green liquor in the causticization reaction of the second step ofthe present invention may be derived from the causticization step of theconventional sulfate or soda process and should be used at 80 to 160 g/L(in which Na₂CO₃ represents 60 to 130 g/L), preferably 100 to 150 g/L(in which Na₂CO₃ represents 85 to 130 g/L) in terms of total alkali. Ifthe total alkali is less than 80 g/L (in which Na₂CO₃ represents 65g/L), the concentration of the final white liquor is lowered and shouldbe regulated before it is used for digestion. If the total alkali ismore than 160 g/L (in which Na₂CO₃ represents 130 g/L), however, theresulting calcium carbonate has a low wire abrasion resistance withoutattaining an intended paper quality.

The green liquor is mixed with said milk of lime prepared in the firststep at a loading rate of the green liquor to the milk of lime of 0.02to 50 cc (green liquor)/min/g (quick lime), preferably 0.02 to 30 cc(green liquor)/min/g (quick lime). Loading rates lower than 0.02 cc(green liquor)/min/g (quick lime) are impractical because of lowproductivity, while loading rate higher than 50 cc (green liquor)/min/g(quick lime) are also impractical because a very high capacity pump isrequired.

Here, the milk of lime prepared from the quick lime in the first stepmay be replaced with a milk of lime prepared from calcium hydroxide atthe same concentration as defined in the present invention.

The causticization reaction should be carried out at a reactiontemperature of 20 to 105° C., preferably 25 to 103° C. Temperatureshigher than 105° C. are uneconomic because the boiling point underatmospheric pressure is exceeded to necessitate a pressure-typecausticization system or the like. If the temperature is lower than 20°C., however, amorphous or massive calcium carbonate is produced tothereby lower the wire abrasion resistance without attaining the desiredpaper quality. This is also uneconomic because the system must bedesigned for cooling, thereby adding to the cost.

Agitation during causticization reaction may be carried out using ameans appropriately selected from conventional agitating blade- orpump-type extruders, kneaders and blenders which can homogeneously mixthe milk of lime prepared in the first step and a green liquor (seeHandbook of Chemical Engineering published by Maruzen, Mar. 18, 1988).

Under the conditions as mentioned above, calcium carbonate in the formof spindle- or rice-like particles having a minimum diameter of 0.3 to1.5 μm and a maximum diameter of 0.5 to 7 μm can be prepared.

Calcium carbonate in various shapes obtained by the present inventiongives better wire abrasion resistance as compared with calcium carbonatepreviously obtained in the causticization step and can be internallyadded to provide fine-quality paper and coated paper with excellentbrightness, opacity, smoothness, writing suitability, sensation oftouch, printability or other properties. From this it can be readilyinferred that it can be used in newspapers, medium papers, printingpapers, book papers, bill papers, dictionary papers, double-side groundwood kraft papers, bleached kraft papers, tissue papers, rice papers,Indian papers, paper boards, non-carbon papers, art papers, light coatedpapers, cast coated papers, wall papers, heat-sensitive papers or thelike to provide them with firm body and excellent brightness, opacity,smoothness, writing suitability, sensation of touch, printability orother properties. It also can be used in various pigments to giveexcellent gloss, smoothness, printabilities or the like. In addition topapers, it can also be used in rubbers, plastics, paints, sealingagents, pressure-sensitive adhesives, fertilizers, etc.

EFFECTS OF THE INVENTION

Although the mechanism of the present invention has not been wellexplained, the calcium carbonate level in the quick lime and the totalalkali level in the white or green liquor seem to have a significantinfluence on the properties of the milk of lime to influence thereaction state between the dissolved calcium hydroxide and carbonateions during the subsequent addition of a green liquor. Sequentialaddition of a green liquor allows the dissolved calcium hydroxide toreact with carbonate ions at a low ratio of carbonate ions during theinitial stage, whereby crystals of calcium carbonate grow into a spindleor rice shape.

This calcium carbonate mainly has two features. Firstly, plastic wireabrasion resistance during high-speed paper making is improved.Secondly, opacity and brightness are improved when it is incorporated.The first feature results from the spindle or rice-like primaryparticles which are more liable to entangle with fibers to improve theyield, thus decreasing the amount of the filler passing through wireparts. The spindle or rice-like particles are also advantageous forimproving abrasion resistance because they have a high aspect ratio andless sharp edges to lower the frictional resistance during contact withwires. The second feature can be explained from electron microscopicobservations of the surface/section of paper, which show that spindle-or rice-like calcium carbonate fills gaps between pulp fibers as if theywere microfibers and were stiff enough to form many minute air spaces toprovide good opacity and brightness.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a scanning electron microphotograph showing the crystalparticle structure of rice-like calcium carbonate obtained in Example 1.

FIG. 2 is a scanning electron microphotograph showing the crystalparticle structure of spindle-like calcium carbonate obtained in Example4.

FIG. 3 is a scanning electron microphotograph showing the crystalparticle structure of amorphous or massive calcium carbonate obtained inComparative example 1.

FIG. 4 is a scanning electron microphotograph showing the crystalparticle structure of rice-like calcium carbonate obtained in Example 7.

FIG. 5 is a scanning electron microphotograph showing the crystalparticle structure of spindle-like calcium carbonate obtained in Example10.

FIG. 6 is a scanning electron microphotograph showing the crystalparticle structure of massive calcium carbonate obtained in Comparativeexample 5.

The following examples illustrate the present invention as compared withcomparative examples without, however, limiting the same thereto.

EXAMPLES Test Method

(1) Alkalinity: determined according to TAPPI 624 hm-85, TAPM625hm-85.

(2) Quick lime particle diameter: determined by a dry procedureaccording to JIS R 9001-1993.

(3) Calcium carbonate content in quick lime: determined from the CO₂level measured by a metal carbon analyzer (EMIA-110 available fromHoriba Ltd.).

(4) Average particle diameter of the product calcium carbonate:determined by a laser diffraction-type particle size distributionanalyzer (Cirrus model 715) after the product was washed with water,filtered, and diluted with water. The minimum and maximum diameters wereactually measured by a scanning electron microscope (JSM-5300 availablefrom JEOL Ltd.) after the product was washed with water, filtered anddried.

(5) Morphology: observed by a scanning electron microscope (JSM-5300available from JEOL Ltd.) after the product was washed with water,filtered and dried.

(6) Crystal system: determined by an X-ray diffractometer RAD-2Cavailable from Rigaku.

Example 1

In a IL four-necked flask (also used in the following examples andcomparative examples), 50 g of a quick lime having a calcium carbonatecontent of 1.6% (and having a particle size distribution of 4.0% 150 μmor more, 18.1% 150-75 μm, 19.4% 75-45 μm, 58.5% 45 μm or less) was mixedwith a white liquor (composition: Na₂CO₃=24 g/L, Na₂S=31 g/L, NaOH=71g/L, all expressed as Na₂O in the same way as in the following examplesand comparative examples) at a quick lime concentration of 7% by weightand then slaked to prepare a milk of lime, which was then causticizedwith a green liquor (composition: Na₂CO₃=110 g/L, Na₂S=34 g/L, NaOH=6g/L, all expressed as Na₂O in the same way as in the following examplesand comparative examples) under the conditions of a green liquor loadingrate of 0.22 cc/min/g (quick lime), loading period of 60 min,temperature of 80° C. and agitation speed of 450 rpm (using POWERSTAIRRER TYPE PS-2N available from KYOEI as an agitator also used in thefollowing examples and comparative examples). As a result ofobservations of the average particle diameter and morphology, thereaction product was found to be rice-like calcium carbonate having anaverage particle diameter of 3.5 μm, an average maximum diameter of 3.5μm and an average minimum diameter of 1.0 μm. Experimental conditionsand results are shown in Table 1.

Example 2

Using 50 g of a quick lime having a calcium carbonate content of 3.0%(and having a particle size distribution of 4.4% 150 μm or more, 17.4%150-75 μm, 20.1% 75-45 μm, 58.1% 45 μm or less) and the same whiteliquor as used in Example 1, the quick lime was mixed with the whiteliquor at a quick lime concentration of 14% by weight and then slaked toprepare a milk of lime, which was then causticized with the same greenliquor as used in Example 1 under the conditions of a green liquorloading rate of 0.5 cc/min/g (quick lime), loading period of 30 min,temperature of 85° C. and agitation speed of 1000 rpm. The product wasfound to be rice-like calcium carbonate having an average particlediameter of 3.6 μm, an average maximum diameter of 3.6 μm and an averageminimum diameter of 1.0 μm. Experimental conditions and results areshown in Table 1.

Example 3

50 g of a kiln-recalcinated quick lime having a calcium carbonatecontent of 7% (and having an average particle diameter 10 mm) was mixedwith the same white liquor as used in Example 1 at a quick limeconcentration of 30% by weight and then slaked to prepare a milk oflime, which was then causticized with the same green liquor as used inExample 1 under the conditions of a loading rate of 28 cc/min/g (quicklime), loading period of 0.5 min, temperature of 95° C. and agitationspeed of 600 rpm. The product was found to be rice-like calciumcarbonate having an average particle diameter of 4.0 μm, an averagemaximum diameter of 4.0 μm and an average minimum diameter of 1.1 μm.Experimental conditions and results are shown in Table 1.

Example 4

The procedure of Example 1 was repeated except that the reactiontemperature during causticization reaction was 30° C. The reactionproduct was found to be spindle-like calcium carbonate having an averageparticle diameter of 5.1 μm, an average maximum diameter of 5.1 μm andan average minimum diameter of 0.7 μm. Experimental conditions andresults are shown in Table 1.

Example 5

The procedure of Example 2 was repeated except that the reactiontemperature during causticization reaction was 40° C. The reactionproduct was found to be spindle-like calcium carbonate having an averageparticle diameter of 4.8 μm, an average maximum diameter of 4.8 μm andan average minimum diameter of 0.7 μm. Experimental conditions andresults are shown in Table 1.

Example 6

The procedure of Example 3 was repeated except that the reactiontemperature during causticization reaction was 50° C. The reactionproduct was found to be spindle-like calcium carbonate having an averageparticle diameter of 4.7 μm, an average maximum diameter of 4.7 μm andan average minimum diameter of 0.6 μm. Experimental conditions andresults are shown in Table 1.

Comparative Example 1

The procedure of Example 3 was repeated except that a quick lime havinga calcium carbonate content of 13% was used. The reaction product wasfound to be amorphous or massive calcium carbonate having an averageparticle diameter of 12.5 μm Experimental conditions and results areshown in Table 2.

Comparative Example 2

The procedure of Example 2 was repeated except that a green liquor(composition: Na₂CO₃=110 g/L, Na₂S=34 g/L, NaOH=6 g/L) was used forslaking. The reaction product was found to be amorphous or massivecalcium carbonate having an average particle diameter of 10.8 μm.Experimental conditions and results are shown in Table 2.

Comparative Example 3

The procedure of Example 3 was repeated except that the reactiontemperature during causticization reaction was 15° C. The reactionproduct was found to be amorphous or massive calcium carbonate having anaverage particle diameter of 9.1 μm. Experimental conditions and resultsare shown in Table 2.

Comparative Example 4

Commercially available heavy calcium carbonate SS-1200 (having anaverage particle diameter 4.4 μm available from Shiraishi Kogyo) wasused.

Example 7

In a 1L four-necked flask (also used in the following examples andcomparative examples), 50 g of a quick lime having a calcium carbonatecontent of 1.6% (and having a particle size distribution of 4.0% 150 μmor more, 18.1% 150-75 μm, 19.4% 75-45 μm, 58.5% 45 μm or less) was mixedwith a green liquor (composition: Na₂CO₃=110 g/L, Na₂S=34 g/L, NaOH=6g/L, all expressed as Na₂O in the same way as in the following examplesand comparative examples) at a quick lime concentration of 30% by weightand then slaked to prepare a milk of lime, which was then causticizedwith a green liquor (composition: Na₂CO₃=110 g/L, Na₂S=34 g/L, NaOH=6g/L, all expressed as Na₂O in the same way as in the following examplesand comparative examples) under the conditions of a green liquor loadingrate of 0.22 cc/min/g (quick lime), loading period of 60 min,temperature of 80° C. and agitation speed of 450 rpm (using POWERSTAIRRER TYPE PS-2N available from KYOEI as an agitator also used in thefollowing examples and comparative examples). As a result ofobservations of the average particle diameter and morphology, thereaction product was found to be rice-like calcium carbonate having anaverage particle diameter of 3.8 μm, an average maximum diameter of 3.8μm and an average minimum diameter of 1.2 μm. Experimental conditionsand results are shown in Table 1.

Example 8

Using 50 g of a quick lime having a calcium carbonate content of 3.0%(and having a particle size distribution of 4.4% 150 μm or more, 17.4%150-75 μm, 20.1% 75-45 μm, 58.1% 45 μm or less) and the same greenliquor as used in Example 1, the quick lime was mixed with the greenliquor at a quick lime concentration of 40% by weight and then slaked toprepare a milk of lime, which was then causticized with the same greenliquor as used in Example 1 under the conditions of a green liquorloading rate of 0.5 cc/min/g (quick lime), loading period of 30 min,temperature of 85° C. and agitation speed of 1000 rpm. The product wasfound to be rice-like calcium carbonate having an average particlediameter of 3.6 μm, an average maximum diameter of 3.6 μm and an averageminimum diameter of 1.2 μm. Experimental conditions and results areshown in Table 1.

Example 9

50 g of a kiln-recalcinated quick lime having a calcium carbonatecontent of 7% (and having an average particle diameter 10 mm) was mixedwith the same green liquor as used in Example 7 at a quick limeconcentration of 50% by weight and then slaked to prepare a milk oflime, which was then causticized with the same green liquor as used inExample 1 under the conditions of a loading rate of 28 cc/min/g (quicklime), loading period of 0.5 min, temperature of 95° C. and agitationspeed of 600 rpm. The product was found to be rice-like calciumcarbonate having an average particle diameter of 4.0 μm, an averagemaximum diameter of 4.0 μm and an average minimum diameter of 1.2 μm.Experimental conditions and results are shown in Table 3.

Example 10

The procedure of Example 7 was repeated except that the reactiontemperature during causticization reaction was 30° C. The reactionproduct was found to be spindle-like calcium carbonate having an averageparticle diameter of 6.1 μm, an average maximum diameter of 1.2 μm andan average minimum diameter of 0.3 μm. Experimental conditions andresults are shown in Table 3.

Example 11

The procedure of Example 8 was repeated except that the reactiontemperature during causticization reaction was 40° C. The reactionproduct was found to be spindle-like calcium carbonate having an averageparticle diameter of 6.8 μm, an average maximum diameter of 1.3 μm andan average minimum diameter of 0.3 μm. Experimental conditions andresults are shown in Table 3.

Example 12

The procedure of Example 9 was repeated except that the reactiontemperature during causticization reaction was 50° C. The reactionproduct was found to be spindle-like calcium carbonate having an averageparticle diameter of 6.7 μm, an average maximum diameter of 1.5 μm andan average minimum diameter of 0.4 μm. Experimental conditions andresults are shown in Table 3.

Comparative Example 5

The procedure of Example 3 was repeated except that a quick lime havinga calcium carbonate content of 13% was used. The reaction product wasfound to be massive calcium carbonate having an average particlediameter of 12.5 μm Experimental conditions and results are shown inTable 4.

Comparative Example 6

The procedure of Example 8 was repeated except that the quick limeconcentration during slaking was 14%. The reaction product was found tobe massive calcium carbonate having an average particle diameter of 10.8μm. Experimental conditions and results are shown in Table 4.

Comparative Example 7

The procedure of Example 9 was repeated except that the reactiontemperature during causticization reaction was 15° C. The reactionproduct was found to be massive calcium carbonate having an averageparticle diameter of 9.1 μm. Experimental conditions and results areshown in Table 2.

Comparative Example 8

Commercially available heavy calcium carbonate SS-1200 (having anaverage particle diameter 4.4 μm available from Shiraishi Kogyo) wasused.

Application Example 1

To a single slurry of hardwood bleached chemical pulp having a Canadianstandard freeness (hereinafter referred to as C.S.F.) of 300 ml wereinternally added 0.02% of a sizing agent (alkyl ketene dimer), 0.5% of asulfate band, 0.3% of cationically modified starch, 15% of each calciumcarbonate obtained in Examples 1 to 6 and Comparative examples 1 to 4and 200 ppm of a yield improver (polyacrylamide having an anionicmolecular weight of 4,000,000 to 5,000,000) was converted into paper bya test machine. The thus obtained paper was conditioned at 20° C., 65%RH for a night and a day, then tested for basis weight, density,brightness and opacity according to JIS. A wire abrasion resistance testwas also performed. Test methods are described below and results areshown in Tables 1 to 4.

Test Method

(1) Wire Abrasion Test

Tester: Nippon Filcon abrasion tester

Wire: Nippon Filcon COS-60 polyester wire

Slurry concentration: 2% by weight

Load: 1250 g

Abrasion period: 90 min

Abrasion wear: wire weight loss after abrasion test (mg).

TABLE 1 Example 1 2 3 4 5 6 CaCO₃ content % 1.6 3.0 7.0 1.6 3.0 7.0Slaking level % 7 14 30 7 14 30 White liquor g/L Na₂CO₃ = 24, Na₂S = 31,NaOH = 71 Green liquor loading 0.22 0.5 28 0.22 0.5 28 rate cc/min/g(quick lime) Caustification 80 85 95 30 40 50 temperature ° C. Agitationspeed, rpm 450 1000 600 450 1000 750 Particle shape rice rice ricespindle spindle spindle Particle diameter μm Average 3.5 3.6 4.0 5.1 4.84.7 Minor 1.2 1.2 1.1 0.7 0.7 0.6 diameter Major 3.5 3.6 4.0 5.1 4.8 4.7diameter Basis weight (g/cm²) 54.1 54.0 54.1 54.2 54.1 54.1 Density(g/cm³) 0.64 0.63 0.64 0.63 0.63 0.63 Brightness (%) 89.7 89.8 89.7 89.789.9 89.9 Opacity (%) 82.7 82.8 82.9 82.8 82.8 82.9 Plastic wire 20 2020 22 22 23 abrasion (mg)

TABLE 2 Comparative example 1 2 3 4 CaCO₃ content % 13 3.0 7.0commercial Slaking level % 30 14 30 heavy CaCO₃ Slaking liquor g/LNa₂CO₃ 24 110 24 Na₂S 31 34 31 NaOH 71 6 71 Green liquor loading 28 0.528 rate cc/min/g (quick lime) Caustification 95 85 15 temperature ° C.Agitation speed, rpm 600 1000 600 Particle shape mass mass mass massParticle diameter μm Average 12.5 10.8 9.1 4.4 Minor diameter Majordiameter Basis weight (g/cm²) 54.1 54.0 54.1 54.1 Density (g/cm³) 0.650.64 0.65 0.64 Brightness (%) 88.0 88.1 87.8 87.9 Opacity (%) 79.3 79.379.4 79.2 Plastic wire 129 125 125 119 abrasion (mg)

TABLE 3 Example 7 8 9 10 11 12 CaCO₃ content % 1.6 3.0 7.0 1.6 3.0 7.0Slaking level % 30 40 50 30 40 50 Green liquor loading 0.22 0.5 28 0.220.5 28 rate cc/min/g (quick lime) Caustification 80 85 95 30 40 50temperature ° C. Agitation speed, rpm 450 1000 600 450 1000 750 Particleshape rice rice rice spindle spindle spindle Particle diameter μmAverage 3.8 3.6 4.0 6.1 6.8 6.7 Minor 1.2 1.2 1.1 0.3 0.3 0.4 diameterMajor 3.8 3.6 4.0 1.2 1.3 1.5 diameter Basis weight (g/cm²) 54.0 54.054.1 54.2 54.1 54.1 Density (g/cm³) 0.64 0.63 0.64 0.63 0.63 0.63Brightness (%) 89.6 89.6 89.5 89.6 89.5 89.4 Opacity (%) 82.6 82.5 82.582.5 82.4 82.5 Plastic wire 23 22 23 22 22 23 abrasion (mg)

TABLE 4 Comparative example 5 6 7 8 CaCO₃ content % 13 3.0 7.0commercial Slaking level % 30 14 50 heavy CaCO₃ Green liquor loading 280.5 28 rate cc/min/g (quick lime) Caustification 95 85 15 temperature °C. Agitation speed, rpm 600 1000 600 Particle shape mass mass mass massParticle diameter μm Average 12.5 10.8 9.1 4.4 Minor diameter MajorDiameter Basis weight (g/cm²) 54.1 54.0 54.1 54.1 Density (g/cm³) 0.650.64 0.65 0.64 Brightness (%) 86.9 87.0 87.0 87.1 Opacity (%) 79.9 79.979.7 79.8 Plastic wire 130 127 125 119 abrasion (mg)

Application Example 2

Each paper prepared in Application example 1 was surface size-pressedwith oxidized starch in a size press to a dry weight of 2 g/m², anddried, then subjected to soft calendering (available from Minamisenju,60° C., constant rate of 50 kg/cm). A 64% coating solution containing60% by weight of heavy calcium carbonate having an average particlediameter of 0.6 μm (tradename: Hydrocarbo 90 available from ShiraishiCalcium), 40% by weight of kaolin having an average particle diameter of0.5 μm (tradename: Ultrawhite 90 available from Engelhard Inc.), 4% byweight of phosphate esterified starch as an adhesive, 10% by weight of astyrene-butadiene latex and 0.3% by weight of a dispersant was appliedon both faces at 10 g/m² per each face by a test blade coater and dried.The thus obtained coated paper was evaluated by the following qualityevaluation test and results are shown in Tables 5 to 8.

Quality Evaluation Method

(1) Glossiness before printing: determined according to JIS P-8142.

(2) Opacity: determined according to JIS P-8138.

(3) Glossiness after printing: determined at an angle of 75° accordingto JIS P-8142 after printing at a constant ink rate of 0.35 cc using anRI printer (Min) with Diatone GSL red available from Sakata Inks.

TABLE 5 Example 1 2 3 4 5 6 Opacity (%) 88 88 88 88 88 88 Glossinessbefore 23 23 23 22 23 22 printing (%) Glossiness after 46 46 45 45 46 46printing (%)

TABLE 6 Comparative example 1 2 3 4 Opacity (%) 85 86 86 86 Glossinessbefore 22 21 21 20 printing (%) Glossiness after 45 44 45 44 printing(%)

TABLE 7 Example 7 8 9 10 11 12 Opacity (%) 88 88 88 88 88 88 Glossinessbefore 23 23 23 22 23 22 printing (%) Glossiness after 46 46 45 45 46 46printing (%)

TABLE 8 Comparative example 5 6 7 8 Opacity (%) 85 86 86 86 Glossinessbefore 22 21 21 20 printing (%) Glossiness after 45 44 45 44 printing(%)

ADVANTAGES OF THE INVENTION

As shown in Examples 1 to 12, calcium carbonate according to the presentinvention was spindle- and rice-like calcium carbonate. The whiteliquors sampled from the step were found to have compositions comparableto conventional conditions.

The results of the paper quality tests of Application example 1 showedthat calcium carbonate of the present invention had high brightness,opacity and plastic wire abrasion resistance.

The coated papers of Application example 2 showed excellent results inopacity.

Moreover, the process of the present invention could greatly reduceproduction costs because it could prepare calcium carbonate with acontrolled shape using the conventional causticization step withoutsignificant change.

What is claimed is:
 1. A process for producing a particulate calciumcarbonate which is suitable as a paper filler by taking advantage of acausticization step of a sulfate process or soda process, comprising:producing a milk or slurry of lime by a first slaking reaction step of(a) adding a white liquor to a quick lime containing 0.1 to 10% byweight of calcium carbonate on the basis of the weight of said quicklime to produce a quick lime mixture until a concentration of said quicklime reaches 0.5 to 60% by weight on the basis of the weight of saidquick lime mixture before slaking; and slaking said quick lime of step(a) with stirring or kneading to prepare said milk or slurry of lime,wherein the quick lime to be added to the first step (a) is obtainedfrom a causticization step of a sulfate process or soda process; asecond causticizing reaction step of sequentially adding to said milk orslurry of lime a green liquor obtained from a causticization step of asulfate process or soda process in an amount necessary to produce awhite liquor and said particulate calcium carbonate having a spindleshape or rice shape, wherein said green liquor is added at a loadingrate of 0.02 to 0.5 cc (green liquor)/min/g (quick lime) and at areaction temperature of 20 to 105° C., and then separating saidparticulate calcium carbonate from said white liquor.
 2. A process forproducing a particulate calcium carbonate which is suitable as a paperfiller by taking advantage of a causticization step of a sulfate processor soda process, comprising: producing a milk or slurry of lime by afirst slaking reaction step of (a) adding a white liquor to a quick limecontaining 0.1 to 10% by weight of calcium carbonate on the basis of theweight of said quick lime to produce a quick lime mixture until aconcentration of said quick lime reaches 0.5 to 60% by weight on thebasis of the weight of said quick lime mixture before slaking; or (b)adding a green liquor to a quick lime containing 0.1 to 10% by weight ofcalcium carbonate on the basis of the weight of a quick lime to producea quick lime mixture until a concentration of quick lime reaches 20 to60% by weight on the basis of the weight of quick lime mixture beforeslaking, and slaking said quick lime of steps (a) or (b) with stirringor kneading to prepare said milk or slurry of lime, wherein the quicklime to be added to the first step (a) or (b) is obtained from acausticization step of a sulfate process or soda process; a secondcausticizing reaction step of sequentially adding to said milk or slurryof lime a green liquor obtained from a causticization step of a sulfateprocess or soda process in an amount necessary to produce a white liquorand said particulate calcium carbonate having a spindle shape or riceshape, wherein said green liquor is added at a loading rate of 0.02 to0.5 cc (green liquor)/min/g (quick lime) and at a reaction temperatureof 20 to 105° C., and then separating said particulate calcium carbonatefrom said white liquor, wherein said particulate calcium carbonate has awidth of about 0.3 μm to 1.5 μm and a length of 0.5 to 7 μm.
 3. Aprocess for preparing calcium carbonate particles having a particulateshape suitable as a paper filler, said process comprising the steps of:adding a white liquor to a quick lime containing 0.1 to 10% by weightcalcium carbonate to obtain a quick lime mixture having a quick limeconcentration of 0.5% to 60% by weight, wherein said quick lime isobtained from a causticization step of a sulfate pulping process or sodapulping process; slaking said quick lime mixture with stirring orkneading to produce a milk or slurry of lime; causticizing said milk orslurry of lime by the addition of a green liquor obtained from acausticization step of a sulfate pulping process or a soda pulpingprocess, said green liquor being added in an amount to produce a whiteliquor and a particulate calcium carbonate having a spindle shape or arice shape at a loading of 0.02 to 0.5 cc (green liquor)/min/g (quicklime) at a reaction temperature of 20° to 105° C.; and separating saidparticulate calcium carbonate from said white liquor.
 4. The process ofclaim 3, wherein said separated particulate calcium carbonate has aparticle size with a width of about 0.3 μm to 1.5 μm and a length ofabout 0.5 μm to 7 μm.
 5. The process of claim 3, wherein said whiteliquor contains 80 to 160 g/l of Na₂O.
 6. The process of claim 5,wherein said white liquor contains 30 g/l or less of Na₂CO₃.
 7. Aprocess for preparing calcium carbonate particles having a particulateshape suitable as a paper filler, said process comprising the steps of:adding a green liquor to a quick lime containing 0.1 to 10% by weightcalcium carbonate to obtain a quick lime mixture having a quick limeconcentration of 20% to 60% by weight, wherein said quick lime isobtained from a causticization step of a sulfate pulping process or sodapulping process; slaking said quick lime mixture with stirring orkneading to produce a milk or slurry of lime; causticizing said milk orslurry of lime by the addition of a green liquor obtained from acausticization step of a sulfate pulping process or a soda pulpingprocess, said green liquor being added in an amount to produce a whiteliquor and a particulate calcium carbonate having a spindle shape or arice shape at a loading of 0.02 to 0.5 cc (green liquor)/min/g (quicklime) at a reaction temperature of 20° to 105° C.; and separating saidparticulate calcium carbonate from said white liquor.
 8. The process ofclaim 7, wherein said separated particulate calcium carbonate has aparticle size with a width of about 0.3 μm to 1.5 μm and a length ofabout 0.5 μm to 7 μm.
 9. The process of claim 7, comprising continuouslyadding said green liquor to said milk or slurry of lime to causticizesaid slaked lime.
 10. The process of claim 9, wherein said quick lime isobtained from a second source.